For health reasons vegetable products--mainly coffee--of reduced caffeine content have gained increasing significance. Thus, quite a number of processes have been developed which, in various ways, try to reduce the caffeine concentration. Normally organic solvents have been used for extraction of the hydrolyzed coffee beans. The recovery of the solvent has been effected by distillation, for example, in the course of which the caffeine is obtained as by-product.
In order to avoid any contamination of the vegetable material by solvent residues, carbon dioxide which is unobjectionable under the health aspect has recently been used as extractant (German Pat. Nos. 2,005,293 and 2,212,281). The carbon dioxide solvent in this process is freed from dissolved caffeine by means of activated carbon.
Prior to recycling of spent activated carbon, the adsorbed substances are normally subjected to pyrolysis whereafter the carbon is thermally reactivated. On account of the usefulness of caffeine, such a procedure is uneconomical.
Hence, it is not surprising that efforts are being made to recover the adsorbed caffeine. Measures for desorbing the caffeine must be selected in consideration of the fact that active carbon is a very good adsorbent, a circumstance that renders desorption difficult. Moreover, the use of any agents that are objectionable under the health aspect is to be avoided because the extraction is effected with carbon dioxide for the very purpose of avoiding such agents.
According to the teaching of German OS No. 2,544,116 the adsorbate is desorbed with supercritical gases, especially carbon dioxide. Thereafter the dissolved adsorbate must be removed from the dissolving gas.
According to U.S. Pat. No. 4,298,736 caffeine adsorbed to activated carbon is desorbed with a food-grade liquid solvent which may be an organic acid or an alcohol. Acetic acid, propionic acid and butyric acid are literally mentioned as organic acids. The process is to be carried out preferably at temperatures above 100.degree. C. and the solvents, i.e., the acids, should have a high boiling point.